With the development of semiconductor technology, the demand for low voltage high current power supplies keeps increasing in order to reduce the power consumption of integrated circuits. When the output voltage of the power supply decreases, the forward voltage of the diode will increase, for example, the forward voltage of a Fast Recovery Diode (FRD) or a Super-fast Recovery Diode (SRD) could be up to 1.0˜2.0 Volts (V). The increment could be approximately 0.6V even for a Schottky diode. As a result, the efficiency of the power supply will be lower. With the development of the synchronous rectifier and corresponding control technology, the application of synchronous rectifying technology is expanding quickly as it helps improve the efficiency, thermal performance, power density, manufacturability and reliability of power supplies.
Depending on the type of the drive signal, a synchronous rectifying function can be realized in two ways: voltage drive and current drive. For the voltage drive, the drive modes of different topologies vary a lot. The application of many topologies is limited and the drive signal is influenced by input voltage. Since a no-current feedback is not applicable in a discontinuous conduction mode (DCM), problems like circulating current will occur when the power supplies are connected in parallel.
For a current drive synchronous rectifier, it obtains the drive signal of synchronous rectifier by detecting current through itself. The synchronous rectifier turns on when detecting the forward current through it and turns off when the current reaches zero, and as a result, the reversed current cannot get through the synchronous rectifier. Like the diode, a synchronous rectifier features unilateral conduction and can be used in various circuitry topologies in power supplying converters. Therefore, the current drive synchronous rectifier has a great future. However, prior art current drive synchronous rectifying technology has many drawbacks such as considerable power consumption, complicated circuitry structure, low working frequency, not being easy to control etc., all of which hinder its application. As shown in FIG. 1, in a current controlled synchronous rectifying drive circuit with energy feedback (as disclosed in U.S. Pat. No. 6,597,587), the current transducer's secondary winding has three windings, two of which are used in energy feedback, and this makes it expensive and complicates the manufacturing process.